average sac rate

[SRQ]

Let me through a kink in - its called compressibility of gases - compressing a gas is non-linear and air or nitrox really gets funny around 3000 psi. The first half of a tank holds more volume air than the last half.

One needs to breathe a whole tank down at a given depth to accurately figure volume used. Thats unless you want to do a ton of math.
Point being there is less cubic feet in a HP 120 tank from 3500 psi to 3000 psi than there is from 2000psi to 1500psi. So what I am saying is I think it is prudent to do an average over several dives using a whole tank doing working and non working dives to establish a baseline air consumption for the two groups and that is how I plan my dives.

Like many above - I find diving solo allows me to have a much more relaxed dive therefore lower SAC rates. Whether I'm in OW or cave diving it has always been noticebly lower.

WB

 

[derwoodwithasherwood]

Let me through a kink in - its called compressibility of gases - compressing a gas is non-linear and air or nitrox really gets funny around 3000 psi. The first half of a tank holds more volume air than the last half.

Interesting point. For air at room temperature I understood the difference at 3,000 psi to be about 3% for air compared to an "ideal" gas. At 4,500 psi, the difference climbs to 10%, IIRC. So it might be a factor for someone with HP tanks, but probably not so much for someone using conventional tanks.

 

[cruiser]

To find your SAC rate:

volume/pressure
80cuft tank/3000 = .02666cuft. This is your tank baseline

Now watch tv or whatever with your reg in your mouth and mask on.
Breath on it for a period of time. Then write down the pressure pressure used.

I'm taking the solo diver course and did this last month to find my SAC rate. It was difficult to breathe through the regulator out of the water, and I got winded several times.

My SAC rate ended up being higher than some of my RMV's.

Anyone else have this experience?

 

[LeadTurn_SD]

I'm taking the solo diver course and did this last month to find my SAC rate. It was difficult to breathe through the regulator out of the water, and I got winded several times.

My SAC rate ended up being higher than some of my RMV's.

Anyone else have this experience?

Interesting observation (about it being more difficult to breath from your reg on the surface).

I know that how a reg breathes on the surface is rarely an indication of how well it will breath underwater (but I'd worry about a reg that breathed "hard" on the surface).

I've noticed subjectively that my regs "feel" better underwater, and seem to breath easier.... my guess would be case fault geometry at play (the effect of water pressure on the 2nd stage diaphragm, relative to the placement of the exhaust valve), combined with the depth-compensating engineered into the 1st and 2nd stages... other folks may have a better answer.

At any rate, my opinion is that the best measure of RMV is taken in the water over a series of dives to get your "average".

Best wishes.

 

[cruiser]

Interesting observation (about it being more difficult to breath from your reg on the surface).

I know that how a reg breathes on the surface is rarely an indication of how well it will breath underwater (but I'd worry about a reg that breathed "hard" on the surface).

I've noticed subjectively that my regs "feel" better underwater, and seem to breath easier.... my guess would be case fault geometry at play (the effect of water pressure on the 2nd stage diaphragm, relative to the placement of the exhaust valve), combined with the depth-compensating engineered into the 1st and 2nd stages... other folks may have a better answer.

At any rate, my opinion is that the best measure of RMV is taken in the water over a series of dives to get your "average".

Best wishes.

After reading through some more material, I think this is what's happening. I have a ScubaPro MK25/S600 and it's designed to have optimal performance UNDER WATER, and it does. Breathing is almost effortless during a dive, even deep dives.

My SAC (sitting in my living room) rate ended up being .302.

Last summer in my quarry in August, the water was about 89 degrees and I had an RMV (underwater) of .27, this was the lowest I had all year, and it was very relaxed diving, nothing strenuous.